CSC 282.01, Class 04: The Bash shell - a refresher

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Questions
• Refresher: Some basic tasks
• What makes a good solution?
• Sample solutions
• Bash

News / Etc.

• Sorry for not getting homework out last week. It was more chaotic than I thought it would be. (It also sounded like some of you needed a break.)

Upcoming Work

Forthcoming

Good things to do

• Today’s CS Extra on the 4-1 program
• Swim meet, all weekend
• Final home basketball games of the year
• Art house event Saturday at 1pm
• Concert Saturday at 2pm with “Sound painting”
• Slavic Coffee House Saturday at 5:30 pm in Bucksbaum Rotunda
• Tuesday’s CS Table on Net Neutrality
• Today, 11am, pay Noteworthy to serenade people
• Monday, bone marrow drive

Questions

Refresher: Some basic tasts

• Given a DOS-formatted text file (lines end with \r\n rather than just \n), convert it to a standard text file.
• Given a standard text file, convert all uppercase letters to lowercase.
• Given a standard text file, remove all blank spaces at the end of lines.
• Make a list of all misspelled words in a text file.
• Given a CSV file in which each line has the form
  LastName,FirstName,Assignment,NumericGrade find the the five highest grades on homework 2.
• Given an HTML file, find the URLs of all images. In case you don’t know HTML, those will typically look like
  <img ... src="*URL*" ...>
  • The img can have any capitalization (img, IMG, Img, iMg, etc.)
  • There can be other text between the img and the src. (That text cannot include a greater than sign.)
  • You may find it easier to start this problem by assuming that there’s only one image tag on a line.

What makes a good solution?

• It works correctly
• As little work for the programmer as possible
• Efficient
• Clear
• Modifiable
• Elegant / clever / not garbage
• You can learn from it

Some sample solutions

Given a DOS-formatted text file (lines end with \r\n rather than just \n), convert it to a standard text file.

• Backstory: Typewriters
• Useful tool: od, octal dump, look at the contents of files
• Solution one: Write a C program
  • Twenty minutes to an hour for student
  • Five minutes Sam (actually, 1:30 for a mediocre version)
• Solution two: In vi, use :1,$s/\r\n/\n/g
  • :1,$ means every line
  • s mean substitute
  • / is the separator
  • \r\n is the pattern
  • \n is the replacement
  • g is “everywhere”
  • Didn’t work
  • Didn’t work with sed. Why not?
  • Debugging: Long time
• Solution three: R (using a scripting language)
  • Load the file into a string
  • Did a search and replace
  • Saved
  • About five minutes
• Solution four: tr - translates or deletes characters
  • tr -d \r
  • One minute

Given a standard text file, convert all uppercase letters to lowercase.

• Solution one: C program
  • Five minutes to fifteen minutes
• Solution two: Python (or other scripting language)
  • Two minutes
• Solution three: tr
  • tr [A-Z] [a-z] < oldfile > newfile
  • tr [:upper:] [:lower:] < oldfile > newfile
  • Why upper and lower? More readable.
  • :upper: and :lower: support i18n.

Given a standard text file, remove all blank spaces at the end of lines.

• Solution one: C program
  • Whoops! Buffer length! (Someone will always find a way to overflow your buffer.)
  • Thirty minutes
• Solution two: vim plugin
  • Install whitespace plugin
  • Save file
  • Five minutes
• Solution three: emacs
  • M-x white-cleanup
  • Five seconds
• Solution four: Scripting language
  • Replace ‘ \n’ with ‘\n’
  • Three minutes
• Solution five: vi search and replace
  • :1,$s/ *$//
  • Ten seconds
• Solution six: sed search and replace
  • sed is “stream editor”, it’s a non-interactive editor. You give it editing commands and it modifies things

Given a CSV file in which each line has the form
LastName,FirstName,Assignment,NumericGrade find the the five highest grades on homework 2.

• Solution one: Load it into Excel, etc.
• Solution two: R or scripting language
  • Read CSV file
  • Subset by things that have column 3 labeled HW2
  • Sort
  • Thirty seconds
• Solution three: C program
• Solution four: cat file | grep ',HW2,' | sort -t, -n -k3 | cut -f4 | head -5 Given an HTML file, find the URLs of all images. In case you don’t